1. Field of the Invention
The present invention is concerned with new isolated wild-type PRRS virus strains and corresponding improved attenuated PRRS viruses, as well as methods of measuring the magnitude of viremia, rate of growth, antibody response, and combinations thereof in such strains and viruses. More particularly, the present invention provides methods for predicting the virulence of new or previously uncharacterized PRRS strains.
2. Description of the Prior Art
Porcine reproductive and respiratory syndrome virus (PRRSV) is an enveloped single stranded RNA virus classified in the family Arteriviridae (Cavanaugh, 1997). It causes a widespread disease of swine that was first described as ‘mystery swine disease’ in the USA in 1987 (Hill, 1990). The disease manifests as respiratory illness in all age groups of swine leading to death in some younger pigs and severe reproductive problems in breeding age females.
The dynamic nature of PRRSV allows for constant change in the disease and provides ample opportunity for the appearance of new strains (Andreyev et al., 1997; Murtaugh et al. 1998; Meng, 2000). The fact that PRRSV changes so readily, coupled with its ability to cause devastating problems for swine producers, makes it an important subject for research (Mengeling et al., 1998; Pejsak et al., 1997) and for the development of vaccines and other methods of reducing the effects of infection. Variation in levels of isolate virulence were demonstrated in lung lesions, and death in swine (Halbur et al., 1996), but efforts to link biological and immunological differences to specific genetic differences has been largely unsuccessful (Albina et al., 1998; Key et al., 2001; Yuan et al., 2001; Murtaugh et al., 2002; Grebennikova et al., 2004). Studies examining the safety and efficacy of PRRS vaccines include the work of Labarque et al., (2003), Mengeling et al., (2003a), and Nodelijik et al. (2001). These studies show that under experimental conditions, modified live PRRS vaccines reduce the amount and duration of viremia as well as fever and lung lesions after virulent challenge.
Opriessing et al. (2002) showed that isolates with high amino acid sequence homology in open reading frame 5 (ORF5) caused significantly different levels of pneumonia in pigs. Variation in swine responses to PRRSV also are affected by host variation (Mengeling et al., 2003b). Virulence has been examined in relation to replication rates and distribution of PRRSV in pigs (Haynes et al., 1997), to macrophage copper clearing capabilities (Thanawongnuwech et al., 1998), and the anemia levels of the host animal (Halbur et al, 2002). However, these methods have been deficient in providing effective methods for predicting the virulence of new or previously uncharacterized PRRS strains.
Accordingly, what is needed in the art is a method of predicting the virulence of PRRS strains. What is further needed in the art is a method of predicting the virulence of PRRS strains based on the rate of in vivo PRRS viral growth and/or viremia magnitude in a swine after administration or exposure of the strain to a previously PRRS-free swine.